Apparatuses for determining at least one parameter of a medium flowing in a conduit in a main flow direction are used, for example, to measure intake air masses of an internal combustion engine. Such apparatuses are used in particular in the form of hot film air mass meters. Other types of apparatus for determining other or further parameters are, however, also conceivable, for example temperature sensors, speed measuring devices, or similar measuring apparatuses, as are measurement principles other than the aforesaid hot film air mass measurement principle.
Hot film air mass meters are discussed, for example, in DE 102 53 970 A1, which refers to an apparatus which has a part that is introducible, with a predetermined alignment with respect to a main flow direction, into a conduit through which the flowing medium is passing.
A partial flow of the medium passes through at least one measurement channel provided in the part, in which channel a measurement element is disposed. Between the inlet and the measurement element, the measurement channel has a curved portion for diverting the partial flow of medium that has entered the measurement channel through the inlet, the curved portion transitioning, as it proceeds, into a portion in which the measurement element is disposed. Provided in the measurement channel is a means that directs the flow and that counteracts flow detachment of the partial medium flow from the channel walls of the measurement channel.
The inlet region is furthermore provided, in the region of the opening that faces away from the main flow direction, with oblique or curved surfaces that are configured in such a way that medium flowing into the inlet region is diverted away from the part of the measurement channel that leads to the measurement element. As a result of this, liquid or solid particles contained in the medium do not, because of their inertia, arrive at the measurement element and cannot contaminate it.
Apparatuses such as, for example, the apparatus in DE 102 53 970 A1 must in practical terms satisfy a number of requirements and boundary conditions. These boundary conditions are largely discussed in the literature, and are discussed, e.g., in DE 102 53 970 A1.
In addition to the objective of decreasing a pressure drop at the apparatuses in global terms by way of a suitable flow-engineering conformation, one of the principal challenges is further to improve the signal quality of such apparatuses. This signal quality refers in particular to the signal swing, which is determined e.g. by a throughput of the medium through the measurement channel leading to the sensor element, and if applicable by a decrease in signal drift and an improvement in signal-to-noise ratio. The configuration, discussed for example in DE 102 53 970 A1, of the inlet opening by way of a surface that deflects liquid and dust particles serves in particular the aforesaid purpose of decreasing signal drift.
At the end of the surface the measurement channel branches off from the main channel, a sharp edge usually being provided here. Water droplets and other contaminants usually cannot follow this detour in the region of the edge, and fly straight through the main channel without arriving at the sensor element. It constantly happens, however, that individual contaminants, in particular water droplets, strike the surface and result in splashes. These lightweight, isolated contamination particles or splashes travel with the measurement air flow into the measurement channel, where they cause signal disruptions. In the case of hot film air mass meters, these signal disruptions are caused in particular by a deposition of liquid films onto the surface of the sensor chip, which modifies the thermal conductivity and heat capacity of the sensor chip, and thus its measurement properties.